Apparatus for measuring and detecting magnetic objects



H. A. KUEHNE 2,587,631 FOR MEASURING AND DETECTING MAGNETIC OBJECTSMarch 4, 1952 APPARATUS 2 Sl-IEETS-SHEET 1 Filed May 21, 1948 ]N VENTOR.

BY ,(QW

f7 7 TOE/VEYS ECTS H. A. KUEHNE March 4, 1952 APPARATU FOR MEASURING ANDDETECTING MAGNETIC OBJ Filed May 21, 1948 2 SHEETSSHEET 2 INVENTOR.

Patented Mar. 4, 1 952 2,587,631 APPARATUS FOR MEAS ING MAGNET URING ANDDETECT- IC OBJECTS Herbert A. Kuehne, Maywood, Ill., assignor toAmerican Can Company, New

York, N. Y., a

corporation of New Jersey Application May 21, 1948, Serial No. 28,423 3Claims. (01. 175-183) The present invention relates to a method of andapparatus for measuring or gauging magnetic objects and has particularreference to a method and device which is particularly adapted to detectmagnetic sheets which vary from a predetermined standard of thickness.

In many industries wherein articles are made from magnetic stock such assheet iron or tin plate, it is necessary to provide devices to measurethe thickness of the sheets before they are fed into printing orfabricating machines in order to prevent the feeding of over-thicksheets which would damage the machines or result in manufacturedarticles of off-standard quality.

At the present time, most of the devices which are utilized to gaugemagnetic sheets for thickness rely upon mechanical feelers which contactthe sheets in order to effect the gauging operation. While such deviceson the whole are fairly satisfactory, they do have inherentdisadvantages which detract from their efiiciency. These disadvantagesinclude inaccuracies due to mechanical wear of the contacting parts, andinaccuracies resulting from contact by the mechanical feeler with minorimperfections such as burred or drip edges or dents in acceptable sheetswhich are of the required thickness but are nevertheless consequentlyrejected as being off-gauge.

The instant invention overcomes these objections by providing a methodof and apparatus for measuring the thickness of magnetic sheet materialswherein the magnetic permeability of the sheets being gauged is utilizedto produce a reliable and highly sensitive gauging operation.

According to this invention, the sheets to be gauged are passed througha detector coil which forms the inductor of a fixed-frequency,constant-potential series resonant circuit. The presence of the magneticmaterial within the detector coil results in an increment in theinductive reactance of the coil with the result that the series resonantcircuit is made to approach resonance. As the circuit approachesresonance, there is a large rise in the voltage across the detector coiland this voltage is utilized as a signal voltage to give a highlyaccurate indication of the sheet thickness. This same efiect will be hadif the magnetic material is brought into proximity to but not passedinto the core of the coil, although the sensitivity of the measuringoperation and apparatus will be somewhat reduced.

The voltage across the inductor of a series resonant circuit atresonance:

E211- fL R where E=voltage applied to circuit fzfrequency in cyclesLzinductance in henrys :effective series resistance of tuned circuit Inthe present invention a fixed-frequency, constant-potential seriesresonant circuit having a fixed capacitor and series resistor isprovided. As the inductance is made to approach its resonance value tothereby tune the circuit towards resonance, the voltages across theinductance coil, across the capacitor and across the series resistorrise sharply. There is also a corresponding sharp rise in the amperagein the circuit.

The voltages across the inductor and the capacitor can both be madeconsiderably greater than the applied voltage when the circuit is in thevicinity of resonance. This is possible because the voltages across theinductor and the capacitor are nearly degrees out of phase with eachother and so add up to a value that is smaller than either voltagealone.

The circuit components are preferably so selected that the greatestpossible variations in the thickness or, in the case of multiple sheets,number of sheets encountered in the normal operation of the apparatus,will produce signal voltages all of which fall within that portion ofthe resonance curve of the circuit wherein the voltage across theinductance coil undergoes its sharpest rise as it rises to its highestvalue (at resonance). This makes it possible to obtain a relativelygreat increment in signal voltage from only a slight increment in sheetthickness. If so desired, the signal voltage increment can be madesubstantially proportional to the sheet thickness increment if asubstantially straight line portion of the curve is utilized.

Care should be taken to avoid creating inductance values exceeding theinductance value 3 at resonance, except in special cases, since thesignal voltages will drop as soon as resonance is passed-and will not bea reliable indication of the increasing thickness of the sheet materialbeing gauged.

The change in the inductive reactance of the resonant circuit is broughtabout by the magnetic properties of the magnetic stock which is placedwithin the center of the detector coil and which functions as a magneticcore. The basic factors in determining these magnetic properties are ofcourse the mechanical dimensions (length, width, thickness) and thephysical and chemical characteristics of the stock which affect itsmagnetic permeability. It thus follows that any one of these basicfactors may be accurately measured by means'of the instant inventionprovided the others remain constant; In order to obtain an accurategauging of the thickness of successive sheets it is necessary that thesheet thickness be affecting the inductance of the detector coil. Hencethe sheets to be gauged should preferably be uniform in length, widthand chemical characteristics. Conversely, it is also possible toaccurately gauge the widthof sheets in which the other mechanicaldimensions and the physical and chemical characteristics remainconstant.

However, reasonable variations in the length of the sheets being passedthrough the detector coil, while not particularly, desirable, areusually permissible since the maximum signal voltage will not beobtained until the sheet is well within the coil. When the sheet is sosituated, its ends are normally considerably removed from the magneticfield of the coil, since in general practice the length of the sheetgreatly exceeds the length of the coil, and does not exert anyappreciable influence on its inductance. It is because of this that thepresent invention is also capable of use with strip or web stock.

It should be apparent that the method and apparatus of this inventionmay be also utilized to measure the magnetic variations due to variablesin the physical and/or chemical characteristics of a given batch ofsheets to determine whether or not they meet metallurgical standards. Inorder to obtain a true signal voltage when this type of measurement isdesired, it is of course necessary that the mechanical dimensions,particularly the thickness and width of the sheets, be uniform.

, An object of the present invention is the provision of a method of andapparatus for electrically gauging magnetic sheet or web stock such astin plate or the like accurately and rapidly without the necessity ofmechanically contacting the materials, thereby precluding the marring orscratching of their surfaces.

' Another object is the provision of an apparatus for measuring thethickness of magnetic sheets wherein the accuracy of the measurement isunaffected by the small imperfections, such as bent or burred edges,dents, or drip edges, which are normally present in a certain percentageof the sheets.

Still another object of the invention is the provision of a sensitiveelectric measuring .and detecting apparatus based upon a series resonantcircuit which can be easily and inexpensively constructed of a fewsimple components and which is easy to maintain.

Yet another object of the invention is to provide a method of aridapparatus. for gauging magthe only variable factor and in their physicalerally by the numeral 22.

4 netic objects wherein the objects to be gauged are brought intoproximity to the inductor of a series resonant circuit in order to alterthe inductive reactance of the circuit.

A further object of the invention is to provide a method of measuringand/or detecting magnetic objects by utilizing a series resonant circuitto produce a signal voltage, the method being adaptable for use in agreat variety of machines such as thickness gauging machines, sortingmachines, double or multiple sheet detectors, and testfiig machines. e

, A still further object is toprovide an electric sheet measuringapparatus which utilizes a differential potentiometer arrangement toindicate the presence of over-thick or multiple sheets.

Numerous other objects and advantages of the invention will be apparentas it is better understood from the following description, which, takenin connection with the accompanying drawings, discloses a preferredembodiment thereof.

Referring now to the drawings:

Figure l is a view, partly diagrammatic and partly in perspective,showing an embodiment of the instant invention which is adapted to givea visual indication of the thickness of successive sheets as they arefed along a path of travel on a.

conveyor.

Figs. 2, 3 and 4 are wiring diagrams illustrating three modified ways inwhich measurements of sheet thicknesses may be obtained from theseriesresonant circuit of Fig. l, and

Fig. 5 is a view, partly diagrammatic and partly in perspective, showingan exemplary embodiment of the instant invention wherein electriccircuits are utilized to stop a conveyor when an over-thick sheet isdetected.

As an exemplary embodiment'of the present invention, Fig. 1 of thedrawings discloses a gauging apparatus wherein magnetic sheets A,substantially uniform in all respects except thickness, are fed along astraight line path of travel on an endless belt or conveyor l0constructed of nonmagnetic material. The conveyor IB- is preferablyoperated continuously, and is so shown, but may, if desired, be of theintermittent type. It is supported on a nonmagnetic table plate II andoperates around a pair of pulleys l2 and i3. Pulley I3 is a drivingpulley, and is driven from a motor l4 through a. pair of drivingsprockets l5, l6 anda drivechain IT.

The sheets A are placed uponthe conveyor l0 in any convenient manner. Asthey are conveyed along in the direction indicated by the arrow in Fig.1, the sheets A pass through a solenoid or detector coil 20. Thedetector coil 20 consists of a suitable number of turns of insulatedwire, and is formed with an open core 2i. The coil 20 closely surroundsthe conveyor l0 and the table plate H, the upper clearance, however,being made sufficient to permit the sheets A to pass unobstructedlythrough the core 2| even though they possess minor imperfections such asbent edges, etc.

The detector coil 20 forms the inductor of a series resonant circuitwhich is designated, gen- The windings of the detector coil 20 areconnected through wires 23, 24 to a source of fixed-frequency,constant-p0! tential alternating current, in this instance shownas a,generator 25. The other components of the series resonant circuitinclude a capacitor or condenser 26 and a series resistor 21.

When the magnetic sheets A pass through the core 2 I of detectorcoil 20,the inductance of the coil is increased, and the circuit 22 is therebymade to closely approach resonance. This results in a sharp rise in thevoltage across the ends of the detector coil 20. A voltmeter 28 isconnected across the ends of the coil 20 and gives a direct visualindication of this voltage, which in this exemplary embodiment of theinvention is utilized as a signal voltage.

By passing sheets of substantially identical dimensions except varyingthicknesses through the coil .20, signal voltage variationscorresponding to the thickness variations of the sheets may be'determined. These values when once known are used by an operator toreject in any manner sheets which are either too thin or too thick. Ifdesired, the voltmeter may be scaled to give a direct reading of thesheet thickness.

As the series resonant circuit 22 approaches resonance, there are markedvoltage rises across the capacitor 23 and across the series resistor 21,in addition to the voltage rise across the inductor 20. While forvarious reasons it is preferable to measure the voltage across theinductor 20, it is also possible to get an indication of the sheetthickness by utilizing the voltage across either the capacitor 26 or theresistor 21 as a signal voltage.

Fig. 2 discloses the series resonant circuit 22 of Fig. 1, wherein asuitable, preferably non-inductive voltmeter 29 is connected across thecapacitor 26.

In Fig. 3, a voltmeter 30 is shown connected across the series resistor21. In each modification the signal voltage readings may be correlatedto the thickness of the sheets passed through the inductor 20 to obtainan accurate index of measurement for the sheet thicknesses. Atresonance, the voltages across the capacitor 26 and across the inductor20 will be substantially equal and may exceed the voltage applied to thecircuit, depending upon the relative magnitudes of the resistance andthe inductive and capacitative impedances. The voltage across theresistor 21 will rise sharply as in the case of the voltages across theinductance coil and the capacitor, to produce a voltage signal but cannever exceed the voltage supplied by the generator 25.

In another modification, shown in Fig. 4, an ammeter 3| is connectedinto the circuit 22 to give a direct reading of the rise in amperage inthe circuit as it approaches resonance. This amperage, as is the casewith the voltages, rises sharply as the circuit approaches resonance andvaries with the thickness of the sheets being measured, and so may beutilized as a signal to give an accurate index of sheet thickness.

Fig. 5 illustrates an embodiment of the present invention wherein thesignal voltage from the detector coil is utilized to stop the operationof a sheet conveyor whenever a multiple sheet or a sheet having athickness which exceeds a predetermined limit is passed through thedetector coil.

This is accomplished by having the signal voltage control the firing oftwo shield grid thyratron tubes 35 and 36 (Fig. 5) in such a manner thatthyratron tube 35 fires whenever a standard sheet is present within thedetector coil and both the thyratron tubes 35 and 36 fire Whenever anover-. thick sheet is present. The firing of thyratron tube 36 resultsin the opening of a circuit which controls the operation of the conveyordrive motor I4 and thus stops the conveyor so that the over-thick sheetcan be removed.

I, .The electric circuits utilized for this thickness gauging operationwill now be explained and reference should be had to the wiring diagramin Fig. 5. A source of alternating current, here shown as a generator40, provides the power for various of the electric circuits through thetwo main lead wires 4|, 42.

The grid bias voltages for the thyratron tubes 35, 36 are obtained bymeans of a full wave rectifier circuit. This circuit includes a fullwave rectifier tube 43 which operates in conjunction with a centertapped transformer 44 to supply a pulsating but continuous outputvoltage. The primary winding 45 of the transformer is connected to theA. C. lead wires 4|, 42 through wires 46, 47.

The secondary winding 48 of the transformer 44 is connected. by wires49, 50 to the rectifier tube 43. Electric current for the heater 55 ofthe rectifier tube 43 is furnished through a transformer 56.

The rectifier circuit also includes a lead wire 51 connected to thecathode 5B of the rectifier tube 43 and a return wire 59 which taps thecenter of the secondary winding 48 0f the transformer 44. A condenser 60is placed across wires 5? and 59 in a cross wire 6| and functions tosmooth out the ripples in the voltage in the rectifier circuit.

The cathodes 62, 63 and the shield grids 64, E5 of the thyratrons 35, 36respectively, are connected into the rectifier circuit wire 5?. Therespective heaters 66, 67 are joined in series and are supplied withelectric current through a transformer 68.

The wire 59 has connected into it suitable resistors "I5, I5 and acontrol potentiometer 'Il having a movable arm 18. The control grid IQof thyratron 35 is connected through a resistor 86 and wire 8! to thecontrol potentiometer arm 78.

A second or diiierential potentiometer 82 having a movable arm 83 isconnected into the wire 8I by means of wires 84, 85. The control grid 88of thyratron tube 36 is connected to the diiferential potentiometer arm83 through a resistor 81 and a wire 88.

It will be apparent from the wiring diagram that in the absence ofsignal voltage the basic grid biases for both the thyratron tubes 35 and36 will be determined by the voltage drop between the setting of thecontrol potentiometer arm 78 and the thyratron cathodes 62 and 63 andwill be substantially the same in both tubes.

In actual operation of this apparatus, however, there will always bepresent a signal voltage to govern the actual grid biases and thus tocontrol the firing of the thyratrons 35, 35.

This signal voltage is supplied by a series resonant circuit generallyindicated by the numeral Hill. The circuit N33 is essentially similar tothe series resonant circuit 22 hereinbefore described in connection withFig. 1. It consists of an inductor or detector coil IBI, a capacitorI52, and a resistor I83, all of which are placed in series and connectedby lead wires I34, I55 to a source of fixed-frequency,constant-potential alternating current, here indicated as a generatorI55.

As shown in Fig. 5, the alternating current for the series resonantcircuit I00 is obtained from a separate source of supply, in this casegenerator IEIB, but it should be understood that as an alternative, thiscurrent can be obtained from the hereinbefore mentioned generator 40. Ifthis is done, the lead wires I04, I05 would be joined to wires 41, 42either directly or through a trans- 7" former, whichever is the moresuitable to the particular circuit requirements.

The solenoid or detector coil II is similar in construction to thepreviously mentioned coil 28, and is formed with an open core I81through which moves a conveyor belt I88 (shown frag mentally) carryingthe sheets A to be measured. The ends of the detector coil IIII areconnected by wires H8, III to the primary winding [I2 of a transformerH3. The secondary winding II4 of the transformer H3 is connected intothe grid control wire BI between the wires 84 and 85. It should beunderstood that the inductance of the transformer windings will affectthe inductance of the series resonant circuit I88 and adequate allowancemust be made therefore in designing the circuit.

The voltage drop across the ends of the detector coil I8I is utilized bythe primary winding ,I I2 as a signal voltage to induce a voltage in thetransformer secondary winding II4 which forms a part of a closed signalcircuit designated as II5, which circuit also includes wires 84 and 85and the differential potentiometer 82.

A selenium rectifier ,6 is placed in the wire 8| in order to prevent thebuilding up of voltages on the grids 19, 88 of the thyratron tubes 35,36 to values above the safety factor of the tubes. To smooth out theripple in the rectified voltage in the signal circuit H5, a condenser H1is placed across the wires 84, 85.

From the above description it will be apparent that as long as currentflows in the series resonant circuit I88, a voltage is induced in thesignal circuit I I which opposes and thus reduces the basic grid biaseson the thyratron tubes 35, 36. This is true even though no sheet bepresent within the detector coil IUI. The full value of this voltage isalways applied to reduce the grid bias of the thyratron tube 35.However, the difierential potentiometer 82 is adjusted so that only apart of this voltage is applied to reduce the grid bias of the thyratrontube 36, thus creating a differential between the grid biases on thethyratrons and 36. It is through this grid bias differential thatselective firing of the thyratron tubes 35, 36 is effected, as willshortly be described.

The anode to cathode circuit of the thyratrons 35, 36 includes atransformer I25, the primary winding H26 of which is connected to theleads 4| and 42. The secondary winding I21 is connected at one end to awire i28 and at the other end to a wire I29. Wire 28 connects with thewire 51 which in turn is connected to the thyratron cathodes 62 and 63.

The plate or anode I30 of the thyratron tube 35 is connected to wire I28by a wire I3I containing an indicating lamp I32. The plate I38 will havean alternating vOltage imposed upon it by the transformer I25 and willbe positive with respect to its cathode 62 only during half of eachcycle.

The plate or anode I33 of the thyratron tube 36 is connected to wire I28through a wire I34 and a resistor I35. In order to make the plate I33constantly positive with respect to its cathode 63, a selenium rectifierI38 is placed in wire I34 and a condenser I31 and a suitable bleederresistor I38 are connected across wire I34 and wire 51.

A description of the manner in which the thyratron tubes 35 and 36- arefired will now be given. As previously explained, the various electricalcircuits shown' in Fig. 5 are set so that in normal'operation thethyratron tube 35- will fire when a standard sheet is fed through thedetec-- tor coil I8I, and both the thyratron tubes 35 and 36 will firewhen an over-thick sheet is fed. In order to determine these settings,the first step is to place a standard sheet within the detector coilI81.

The resonant circuit I88 is thereby made to approach resonance, with theresult that the signal voltage is greatly increased and the grid biasvoltages on both the thyratron tubes 35 and 36 are reduced. The movablearm 18 of control potentiometer'11 is now adjusted to further reduce thegrid bias voltages on both thyratrons until thyratron 35 fires, therebylighting the indicator light I32. Electric current now flows from theplate I38 to the cathode 62 of thyratron 35, and thence through wires 51and I29, the transformer winding I21, wire I28, and wire I31 andindicator lamp I32 back to the plate I38.

If the full voltage which is induced in the signal circuit when astandard sheet is present in the detector coil I81 were applied againstthe grid bias of the thyratron tube 36, it would fire simultaneouslywith the thyratron tube 35. The

thyratron tube 36, however, does not fire when a standard sheet ispresent within the detector coil I8! because the differentialpotentiometer 82 makes only a portion of this voltage available to it toreduce its grid bias.

The thyratron 36 can therefore be made to fire only when a higher signalvoltage is supplied by the detector coil Hill to compensate for theunused portion of the induced voltage in the sisnal circuit due to thediiferential potentiometer 82. This increase in signal voltage isproduced as the resonant circuit 588 is made to more nearly approachresonance by an increase in the thickness of the magnetic sheet withinthe coil.

The voltage increment or difierential necessary to cause the thyratron36 to fire is determined by the setting of the difierentialpotentiometer 82. Hence, the standard sheet is removed from the detectorcoil IOI and the potentiometer 82 is now set to permit the thyratron 36to fire whenever a sheet which exceeds a maximum allowable thickness ispresent within the detector coil NH. The thickness increment betweenacceptable and non-acceptable sheets may thus be varied by a simpleadjustment of the diiferential potentiometer 82.

'When the thyratron tube 36 fires as a result of the presence of anover-thick sheet in the detector coil I8I, current flows from its anodeI33 to its cathode 63, and thence through wires 51 and I29, transformerwinding I21, and wires I28 and I34 back to the anode I33. This flow ofcurrent is utilized to operate a double wound control relay I48 to stopthe conveyor I88 so that the over-thick sheet may be removed.

The relay I48 includes a winding I4I which is connected between wires 51and 59 in the rectified circuit, and thus is continuously keptenergized. When so energized, winding I4I attracts and holds the relayarmature I42 in its normal operative position (to the left as shown inFig. 5) against the pressure of a compression spring I43. In thisposition, the outer switch I44 of the. relay is closed, thus maintainingthe flow of current in the motor circuit generally designated by thenumeral I45, which circuit controls the motor which drives the sheetconveyor I88. The conveyor is thereby kept in operation and a constantflow of sheets A is insured.

A second relay winding I46, which is placed in wire I34 and whichopposes winding MI, is

energized when the thyratron tube 36 fires, with the result that themagnetic eflect of winding I41 is neutralized and the spring I43 movesthe relay armature I 42 to the right (as indicated. in broken lines inFig. 5) thus opening the switch I44 and stopping the conveyor.

This movement of the relay armature I42 also closes an inner switch I41,resulting in the flow of current through a wire I50 which is connectedacross wires I28 and 51. An indicating lamp I5I, placed in wire I58furnishes a visual-indication of the firing of the thyratron tube 36 andthus of the presence of an over-thick sheet within the detector coilIDI.

As hereinbefore explained, the thyratron 35 has alternating voltage onits plate I35, and thus will cease to conduct as soon as the magneticsheet which caused it to fire is Withdrawn from the detector coil Illl.The plate I33 of thyratron 36 is, however, always positive with respectto its cathode B3, and thus, when once fired, will continue to conductuntil the plate voltage is sufiiciently reduced. To make possiblethis'reduction, a wire I52 containing a normally open reset push buttonI53 is placed across wires 5? and I34.

To break the anode-to-cathode circuit of thyratron 33 and restart theconveyor after the over-thick sheet has been removed, the push buttonI53 is first closed to destroy the voltage on plate I33, and thus stopthe flow of current through the thyratron 36. The current now bypassesthe thyratron 36 through wire I52 and continues to flow through therelay winding MS.

The push button I53 is next released, thus breaking the flow of currentthrough the relay winding I 46 and permitting the relay winding I4I tomove the armature I 42 to the left, thus closing the switch [44 andrestarting the conveyor motor, and opening the switch I41 to extinguishthe indicating lamp I5I.

It will be apparent from the foregoing description that the measuringand detecting apparatus illustrated in Fig. 5 gives an indication of thethickness of each sheet which passes through the detector coil II] I.Neither of the thyratron tubes 35, 36 fires when a sheet which is ofless than standard thickness is present within the detector coil, withthe result that neither of the indicating lamps I32, ISI lights. Such asheet is allowed to continue on since it will not cause damage tosubsequent machinery.

The presence of a standard sheet is indicated when only the lamp I32lights, while an overthick sheet causes both the lamps I32 and I53 tolight and also results in the breaking of the motor control circuit ofthe sheet conveyor and the consequent stopping of the conveyor.

The apparatus is of course perfectly adapted to function as a multiplesheet detector. When a-plurality of sheets are fed simultaneously, theyhave an effect upon the inductance of the series resonant circuit Isimilar to that of an overthick sheet, with the result that both thelamps I32 and II light and the conveyor is stopped.

It is thought that the invention and many of its attendant advantageswill be understood from the foregoing description, and it will beapparent that various changes may be made in the form, construction, andarrangement of parts of the apparatus mentioned herein and in the stepsand their order of accomplishment of the method described herein,without departing from the spirit and scope of the invention orsacrificing'all of its material advantages, the apparatus and methodhereinbefore described being merely a preferred embodiment thereof.

I claim:

1. An apparatus for measuring the thickness of magnetic stock,comprising a series resonant circuit including an inductor in the formof an open-core detector coil for supplying a signal voltage, themagnitude of which varies with the thickness of the magnetic stock whensaid stock is passed directly through the open core of said detectorcoil, conveyor means for moving said stock through said open core ofsaid detector coil, a pair of thyratron tubes associated with said serieresonant circuit, means for impressing on the grid bias of each of saidthyratron tubes a voltage whose magnitude is responsive to the variationin said signal voltage, means for adjusting the grid bias on the firstof said thyratron tubes so that it will conduct when material which isat least of standard thickness is moved through said detector coil, andmeans for adjusting the grid bias of the second of said thyratron tubesso that it will conduct only when material which exceeds thepredetermined standard of thickness is moved through said detector coil.

2. An apparatus for measuring thickness of magnetic stock, comprising aseries resonant circuit including an inductor in the form of anopen-core coil for supplying a signal voltage, the magnitude of whichvaries with the thickness of the magnetic stock when said stock ispassed directly through the open core of said detector coil, conveyormeans for moving said stock through said open core of the detector coil,a first thyratron tube and a second thyratron tube, means for imposingsaid signal voltage onto the control grid of said first thyratron tubeand a differential signal voltage onto the control grid of said secondthyratron tube, means for conducting an alternating current to the anodeof said first thyratron tube and a rectified current to its cathode, asignal device in the exterior anode to cathode circuit of said firstthyratron tube said signal device being operable by current passingthrough the circuit when conduction in said first tube is effected bysignal voltage created by stock of predetermined thickness passingthrough said open-core coil, means for applying rectified current toboth the anode and the cathode of said second thyratron tube, conveyorcontrol means in the exterior anode to cathode circuit of said secondthyratron tube for controlling operation of said conveyor, saiddiiferential signal voltage imposed upon the control grid of said secondthyratron tube being sufficient to produce conduction in the tube forclosing its anode to cathode circuit to initiate operation of saidcontrol means when the magnetic stock passing through said open-corecoil exceeds said predetermined thickness, and means for discontinuingconduction in said second tube to open its anode to cathode circuitafter removal of said stock of excessive thickness from said open-corecoil.

3. An apparatus for measuring the thickness of magnetic stock,comprising a series resonant circuit, a first thyratron tube and asecond thyratron tube associated with said resonant circuit, measuringmeans including an open inductor coil in said resonant circuit forsupplying to both of said thyratron tubes a signal voltage the magnitudeof which varies in accordance with variations in the thickness of thestock, means for conveying said stock directly through said openinductor coil, a control potentiometer .for adjusting the basic gridbiases on both of said thyratron tubes, a diiferential potentiometer forfurther adjusting the grid bias on said second thyratron tube so thatthe latter conducts only in response to a greater signal voltage than isrequired to cause said first thyratron tube to conduct, a conveyorcircuit having a conveyor control switch connected therewith forstopping and starting said conveyor, an indicator circuit having anindicating device and an indicatorswitch connected in series therein, acontrol relay having a first and a second winding and an armature unitedwith said switches, said armature being mechanically biased to maintainsaid conveyor control switch open and said indicator switch closed, saidfirst winding being energized continuously for biasing said controlarmature in opposition to said mechanical bias for maintaining saidindicator switch open and said conveyor control switch closed whilecurrent is being supplied to the apparatus, said second winding beingconnected in series in the exterior anode to cathode circuit of saidsecond thyratron tube whereby, upon conduction in said second tube,

- .said second winding is energized for neutralizing the effect of saidfirst winding so that themechanical bias of said armature opens saidconveyor control switch and closes said indicator switch for stoppingsaid conveyor and for energizing said indicating device when over-thickstock is present in said inductor coil.

HERBERT A. KUEHNE.

REFERENCES CITED UNITED STATES PATENTS Number Name Date 1,640,524Augustine Aug. 30, 1927 1,943,619 Mudge et a1 Jan. 16, 1934 1,946,924Allen et a1. Feb. 13, 1934 1,957,222 Mershon May 1, 1934 1,983,388 MooreDec. 4, 1934 2,071,607 Bjorndal Feb. 23, 1937 2,222,221 Burford Nov. 19,1940 2,228,294 Wurzbach Jan. 14, 1941 2,312,357 Odquist et a1 Mar. 2,1943 2,331,418 Nolde Oct. 12, 1943 2,349,992 Schrader May 30, 1944

